COMBINATION BEARING CONSTRUCTION

Abstract: A combination bearing construction capable of withstanding high-speed rotation and having a high accuracy of rotation is provided. The combination bearing construction comprises a first bearing means, a second bearing means, a third bearing means and a fourth bearing means. The first bearing means bears impulsive forces in a radial direction, which impulsive forces are exerted on a rotary body during rotation, and comprises an inner race (1) and an outer ring (2) which are formed of a silicon nitride based ceramic sintered body. The second bearing means bears a load in an axial direction with a predetermined gap maintained between it and the rotary body, which load is exerted on the rotary body, and is constituted by a magnetic bearing body which comprises two permanent magnets (12, 13) opposing each other in a thrust direction. The third bearing means serves to maintain an accuracy of rotation in a radial direction of the rotary body, and comprises radial dynamic pressure generating grooves (5) formed on a cylindrical surface of the inner race (1). The fourth bearing means serves to maintain an accuracy of rotation in a thrust direction of the rotary body, and comprises an air reservoir (100) confined in a semi-closed condition by the inner race (1), outer ring (2) and a cover (4).

Description: SPECIFICATION Technical Field The present invention generally relates to a bearing structure, and more specifically, it relates to a composite bearing structure, having a prescribed load, for supporting a rotator rotating at a high speed. Background Technique With increase in storage capacity and reduction in access time of a hard disk driver (hereinafter referred to as "HDD"), a high rotational speed and high rotational accuracy corresponding thereto are required for a spindle motor for driving the HDD in recent years. In a sliding part of this rotating apparatus, a ball bearing has generally been employed for a bearing part, for example. So far as a conventional ball bearing has been employed, however, the upper withstandable rotational speed has been about 10000 r.p.m., due to a problem of seizure or wear. In order to drive a precision motor requiring such a high rotational speed and high rotational accuracy at a higher speed, there has been proposed employment of an air bearing (dynamic pressure gas bearing) of ceramics such as SiC or Si 3 N 4 for a rotary sliding part. When a rotator is driven in a rotating apparatus employing such an air bearing, air is forcibly introduced at least into a clearance between a radial bearing body or a thrust bearing body and the rotator through a groove. Thus, the air pressure in the clearance is increased and the rotator is driven at a high speed through an air bearing which is provided between the respective members. In order to implement high-speed rotation in such a manner, rotational accuracy during high-speed rotation is maintained by the air bearing, and a thrust-directional load which is applied to the rotator is supported by the air bearing. Further, the aforementioned radial bearing body and the rotator can withstand sliding between the respective members in a low-speed area in starting/stopping of the rotator since the same are formed by ceramics materials respectively. When a conventional radial bearing body which is formed by ceramics such as Si 3 N 4 is employed in high-speed rotation exceeding 5000 r.p.m., however, an impactive knock wear phenomenon is caused by contact between bearing members. Namely, sliding between the respective members is caused in a high-speed area when abrupt radial impact force is applied to the rotator in high-speed rotation. It has been difficult for a conventional ceramics sintered body to withstand the high-speed sliding. Even if a radial bearing body or a thrust bearing body has been formed by an air bearing body consisting of a ceramics sintered body as described above, therefore, it has been difficult to support radial impact force abruptly applied to a rotator in high-speed rotation, although rotational accuracy in high-speed rotation has been maintained and a thrust-directional load applied to the rotator has been supported. When the aforementioned air bearing member is made of a ceramics material, further, high working/assembling accuracy is required for the ceramics member to lead to increase in manufacturing cost. Further, it is necessary to use the air bearing under clean environment since the same must be prevented from penetration of dust. Thus, a structure accompanying the air bearing is complicated such that the air bearing is employed in a state stored in a closed vessel, and hence the storage space is disadvantageously increased in size. In order to solve such a problem, it may be possible to employ a bushing type slide bearing body which is made of ceramics as a bearing body for high-speed rotation. Fig. 11 is a longitudinal sectional view showing a schematic structure of a conventional bushing type slide bearing body which is made of ceramics. This bushing type slide bearing body is formed by two thrust slide bearing bodies 81 and 82, and pairs of radial slide bearing bodies and rotators 83 and 84. The bushing type slide bearing body having such a structure can withstand sliding between the respective members in a low-speed area in starting/stopping. In sliding following high-speed rotation exceeding 5000 r.p.m., however, an impactive knock wear phenomenon is caused by contact between the bearing bodies, similarly to the aforementioned air bearing body of ceramics. Thus, it has been difficult for the conventional bushing type slide bearing body to withstand sliding such that frictional resistance has been disadvantageously increased due to roughening of sliding surfaces. This tendency is remarkable with respect to a thrust-directional load, in particular. Comparing through the same load, thrust-directional frictional resistance is larger by 5 to 10 times as compared with radial frictional resistance. Even if a bushing type slide bearing body of ceramics has been employed as a bearing body for high-speed rotation, therefore, it has been difficult not only to support abrupt radial impact force during rotation, but to support a thrust-directional load which has been applied to a rotator. In order to solve such a problem, the inventors have invented a composite bearing structure described in Japanese Patent Application No. 3-298841 and the corresponding European patent application EP-A-0 528 274, forming prior art in accordance with Article 54(3) EPC. According to the inventors, however, it has been proved difficult to maintain thrust-directional rotational accuracy with respect to impact or vibration from the exterior in the aforementioned composite bearing structure. When impact has been applied to a structural body including a bearing body from the exterior during high-speed rotation, for example, a rotor has vibrated in an axial direction and a convergence time thereof has been disadvantageously increased. EP 0 178 169 A2 discloses a rolling element bearing having members made of silicon powder, silicon nitride powder, and silicon carbide powder by roasting the resulting mixture in an atmosphere of nitrogen at higher temperatures. Further, JP-A-4031694 discloses a composite bearing structure according to the preamble of claim 1 comprising axial bearing means including a gas dome portion being confined in a semi-closed state by a rotating member rotating with a rotator and a fixed member being axially opposed to the rotating member for maintaining thrust-directional rotational accuracy of the rotator. Accordingly, it is an object of the present invention to provide a composite bearing structure providing an improved resistance against wear phenomena. According to the present invention, this object is solved by providing a composite bearing structure having the features as defined in claim 1. Preferred embodiments of the invention are defined in the dependent claims. Even if abrupt radial impact force is applied to the rotator which rotates at a high speed, the ceramics sintered body included in the radial bearing means supports the impact force, and can withstand sliding in a high speed area in the present invention. Further, an axial (thrust-directional) load which is applied to the rotator is supported by the axial bearing means in a state keeping a clearance between the same and the rotator. Even if the rotator rotates at a high speed, therefore, an increasing rate of frictional resistance which is caused with respect to the thrust-directional load can be reduced. Further, radial rotational accuracy of the rotator rotating at a high speed is maintained by the radial bearing means. The axial bearing means prevents thrust-directional movement of the rotator which is caused by impact or vibration applied from the exterior. Therefore, rotational accuracy which is required for a precision machine can be maintained also after a long-time operation, As hereinabove described, the radial and axial bearing means are provided according to the present invention, whereby no wear phenomenon is caused by abrupt impact force during high-speed rotation and increase of frictional resistance caused by a thrust-directional load which is applied to the rotator is suppressed, while rotational accuracy can be maintained also in long-time rotation. According to the present invention, therefore, it is possible to provide a structure of a composite bearing body which is suitable for supporting a rotator rotating at a high speed in high rotational accuracy. Further, the ceramics sintered body included in the radial bearing means includes a silicon nitride ceramics sintered body. Further, this silicon nitride ceramics sintered body contains crystal grains having linear density of at least 35 per length of 30 µm, and a volume rate of grain boundary phases thereof is not more than 15 volume %. In addition, the sintered body of silicon nitride contains pores of not more than 20 µm in maximum diameter, and the content of the pores is not more than 3 %. Such a silicon nitride sintered body at least has mechanical properties of at least 80 kg/mm 2 , preferably at least 100 kg/mm 2 as JIS three-point bending strength, and at least 5 MPa·m ½ as a fracture toughness value. More preferably, JIS three-point bending strength is improved to at least 140 kg/mm 2 , and wear resistance and impact resistance strength for serving as a bearing member are improved to at least twice those of a sintered body having linear density of 40, in the sintered body in which the linear density of crystal grains per length of 30 µm is improved to at least 50. In the preferred first aspect of the present invention, the sintered body of silicon nitride forming the first bearing means contains crystal grains having linear density exceeding a constant value, while the volume rate of grain boundaries is suppressed below a constant value and porosity is suppressed below a constant value. Thus, the bearing member is formed by a sintered body of silicon nitride comprising fine crystal grains. Consequently, the surface of the silicon nitride sintered body forming the bearing member is not chipped by falling of the crystal grains etc., but the same is also excellent in chipping resistance. Thus, the bearing member consisting of the aforementioned silicon nitride sintered body has excellent wear resistance also with respect to high-speed sliding accompanied with a knock wear phenomenon. Therefore, abrupt radial impact force which is applied to the rotator during high-speed rotation is supported by the aforementioned silicon nitride sintered body, and the aforementioned silicon nitride sintered.body exhibits excellent wear resistance also with respect to high-speed sliding caused at this time. Further, the axial bearing means includes a gas dome portion which is confined in a semi-closed state by a rotating member rotating with the rotator and a fixed member axially opposed to the rotating member. When entrance/discharge of air is not smoothly performed through a radial clearance in the interior of the bearing, the pressure in the gas dome portion fluctuates by impact or vibration from the exterior. Even if the rotator is driven by the impact or vibration from the exterior to move in the thrust direction, the pressure fluctuation in the gas dome portion acts in a direction for inhibiting the rotator from thrust-directional movement in this case. Thus, the rotator is inhibited from thrust-directional movement. Consequently, the amount of thrust-directional movement of the rotator, the width of thrust-directional vibration of the rotator and a convergence time thereof are reduced as compared with the prior art, whereby thrust-directional rotational accuracy is improved. The semi-closed state is now described. Air in the gas dome portion can be supplied or discharged through the clearance in the bearing. However, air cannot be smoothly supplied or discharged as the radial clearance in the bearing is reduced. Therefore, supply or discharge of air will not follow even if the rotator is to move in the thrust direction in response to impact or vibration which is applied from the exterior, and the pressure in the gas dome portion is changed. The gas dome portion in such a state is called a gas dome portion confined in a semi-closed state. The aforementioned phenomenon remarkably starts to appear when the radial clearance in the bearing is at a degree of about 50 µm. A bearing which is employed for a rotating apparatus such as an HDD has a clearance of not more than 20 µm. Therefore, an effect based on the aforementioned phenomenon can be sufficiently applied to a bearing for a driving motor of an HDD or the like in practice. Further, the rotating member for confining the gas dome portion includes first and second rotating portions. The first rotating portion is opposed to the fixed member while radially keeping a clearance. The second rotating portion, which is opposed to the fixed member while axially interposing the gas dome portion, is fixed to the first rotating portion. When the radial or axial bearing means is formed by dynamic pressure gas bearing means, the second rotating portion has a hole. A gas which is introduced into the prescribed clearance by the dynamic pressure gas bearing means is discharged through the hole provided in the second rotating portion. In this case, the rotator is held in a position balancing the pressure in the gas dome portion and driving force which is caused by the air discharged from the hole of the second rotating portion. At this time, it is necessary to reduce the diameter of the hole of the second rotating portion to a degree for causing pressure fluctuation in the gas dome portion. Thus, entrance/discharge of air in the bearing is not smoothly performed even if impact or vibration is applied to the rotator from the exterior and the rotator is to move in the thrust direction. Therefore, the pressure in the gas dome portion fluctuates and force acts in a direction for returning the internal pressure to the original level. Consequently, the rotator is thrust-directionally pushed back to the original position. Preferably the axial bearing means includes dynamic pressure gas bearing means which supports a load by a gas pressure produced by rotation of the rotator. Further, the axial bearing means includes magnetic bearing means which supports the load through magnetic attraction or repulsion. Each of these dynamic pressure gas bearing means and magnetic bearing means includes a rotating member which rotates with the rotator and a fixed member which is opposed to the rotating member while axially keeping a clearance. In the dynamic gas bearing means, the rotating member and the fixed member include ceramics sintered bodies, and either one of the rotating member and the fixed member is provided with a groove for introducing a gas into the clearance for producing the gas pressure. In the magnetic bearing means, the rotating member and the fixed member include magnets for producing magnetic attraction or repulsion in the clearance. Thus, a thrust-directional load which most greatly contributes to increase of frictional resistance of the bearing body can be supported or reduced by the dynamic pressure gas bearing means through the pressure of the gas which is introduced into the clearance from the groove. Or, the thrust-directional load can be supported or reduced by the magnetic bearing means through repulsion or attraction of the magnets. Thus, increase of a driving torque can be suppressed extremely small with respect to increase of the rotational speed. Particularly when the axial bearing means includes the magnetic bearing means, increase of the driving torque with respect to increase of the rotational speed is suppressed smaller as compared with the dynamic gas bearing means. Preferably, the radial bearing means includes a ceramics sintered body for maintaining rotational accuracy in a state keeping a prescribed clearance with the rotator. In this case, it is possible in principle to improve the rotational accuracy by reducing the clearance as narrow as possible. Preferably, the radial bearing means includes dynamic pressure gas bearing means which maintains rotational accuracy by a gas pressure produced by rotation of the rotator. The dynamic pressure gas bearing means includes a rotating member which rotates with the rotator, and a fixed member which is opposed to the rotating member while radially keeping a clearance. The rotating member and the fixed member include ceramics sintered bodies, and either one of the rotating member and the fixed member is provided with a groove for introducing a gas into the clearance for producing the gas pressure. When dynamic pressure gas bearing means is thus employed in the radial bearing means, rotational accuracy can be maintained in high accuracy in long-time high-speed rotation, even if the clearance between the rotating member and the fixed member is relatively increased. Preferably, further, the radial bearing means includes magnetic bearing means for maintaining rotational accuracy through magnetic attraction or repulsion in a state keeping a prescribed clearance with the rotator. The magnetic bearing means includes a rotating member which rotate with the rotator, and a fixed member which is opposed to the rotating member while radially keeping clearance. The rotating member and the fixed member include magnets for producing magnetic attraction or repulsion in the clearance. The rotating member and the fixed member consist of ceramics sintered bodies, and the magnets are buried in the interior of the ceramics sintered bodies. Also when magnetic bearing means is thus employed in the radial bearing means, rotational accuracy can be maintained in high accuracy even if high-speed rotation is continued for a long time in a state of relatively increasing the clearance between the rotating member and the fixed member. According to the present invention, as hereinabove described, it is possible to provide structures of various composite bearing bodies which are suitable for supporting rotators rotating at high speeds, and excellent in impact resistance and vibration resistance. Brief Description of the Drawings Fig. 1 is a conceptual diagram schematically showing a structural example A of a composite bearing body according to the present invention. Figs. 2A and 2B are conceptual diagrams schematically showing two types of a structural example B of the composite bearing body according to the present invention. Fig. 3 is a conceptual diagram schematically showing a structural example C of the composite bearing body according to the present invention. Fig. 4 is a conceptual diagram schematically showing a structural example D of the composite bearing body according to the present invention. Figs. 5A and 5B are conceptual diagrams schematically showing two types of a structural example E of the composite bearing body according to the present invention. Fig. 6 is a conceptual diagram schematically showing a structural example F of the composite bearing body according to the present invention. Figs. 7A and 7B are graphs showing the relations between wear rates and P•V values of a sintered body forming a radial impact resistance member in a composite bearing structure according to the present invention with those of conventional and comparative examples. Fig. 8 is a schematic diagram showing the structure of a spindle motor employed for evaluating bearing performance of the structural examples C and F of the composite bearing body according to the present invention. Fig. 9A is a schematic diagram showing Example (A) as a bearing employed for evaluating bearing performance of the structural example F of the composite bearing body according of the present invention, and Figs. 9B and 9C are schematic diagrams showing structures of comparative examples (B) and (C). Fig. 10A is a schematic diagram showing Example (A) as a bearing employed for evaluating bearing performance of the structural example C of the composite bearing body according of the present invention, and Figs. 10B and 10C are schematic diagrams showing structures of comparative examples (B) and (C). Fig. 11 is a longitudinal sectional view schematically showing the structure of a conventional bushing type slide bearing. Best Modes for Carrying Out the Invention Various structural examples of the inventive composite bearing body are shown in Table 1. Table 1 shows structural examples of A to F as combinations of radial bearing means (first and third) and thrust bearing means (second). Schematic structures of the respective structural examples are shown in Figs. 1 to 6 in correspondence to A to F. With reference to Table 1 and the respective figures (Figs. 1 to 6), embodiments of structures of composite bearing bodies are now described in order. Radial Bearing Means (first and third) Ceramics Air Bearing Magnetic Bearing + Ceramics Bushing Ceramics Bushing Thrust Bearing Means (second) Ceramics Air Bearing A (Fig. 1) B (Figs. 2A and 2B) C (Fig. 3) Magnetic Bearing D (Fig. 4) E (Figs. 5A and 5B) F (Fig. 6) Structural Example A Fig. 1 is a conceptual diagram schematically showing a composite bearing structure according to an embodiment of the present invention. As shown in Fig. 1, the composite bearing structure comprises an inner ring 1, an outer ring 2, a lower thrust plate 3 and a cover 4. The cover 4 is bonded to the outer ring 2. The respective members 1 to 3 consist of silicon nitride ceramics sintered bodies. The inner ring 1 has a cylindrical shape, to be capable of enveloping an outer peripheral surface of a prescribed shaft. A plurality of V-shaped radial dynamic pressure producing grooves 5 are formed in the cylindrical surface of the inner ring 1. The lower thrust plate 3 is provided to be in contact with an end surface of the inner ring 1. The outer ring 2 having a cylindrical shape is provided to have prescribed clearances with respect to the cylindrical surface of the inner ring 1 and an inner side surface of the lower thrust plate 3. A plurality of spiral thrust dynamic pressure producing grooves 6 are formed in surfaces of the outer ring 2 and the lower thrust plate 3 which are opposed to each other. While the thrust dynamic pressure producing grooves 6 are formed in both of the outer ring 2 and the lower thrust plate 3, the same may be formed in either one. Further, the cover 4 is provided with a small air vent hole 101. This air vent hole 101 is adapted to bleed air which is introduced along the dynamic pressure producing grooves in rotation. A portion enclosed with the inner ring 1, the outer ring 2 and the cover 4 defines an air dome portion 100 which is confined in a semi-closed state. In the composite bearing body structured in the aforementioned manner, it is assumed that the outer ring 2 and the cover 4 rotate with a rotator, and the inner ring 1 and the thrust plate 3 are fixed to the prescribed shaft. When the outer ring 2 rotates with the rotator, air is introduced between the outer ring 2 and the inner ring 1 and between the outer ring 2 and the lower thrust plate 3 along the radial dynamic pressure producing grooves 5 and the thrust dynamic pressure producing grooves 6, so that the clearances between the same are kept constant respectively. Thus, an air bearing is defined. The outer ring 2 rotating with the rotator can be driven at a high speed without being in contact with the inner ring 1 and the lower thrust plate 3, by air pressures which are produced in the respective clearances. According to the structural example A, both of a radial bearing body and a thrust bearing body are formed by ceramics air bearing bodies. Therefore, radial rotational accuracy is maintained by the radial air bearing, while a thrust-directional load is supported by the thrust air bearing body. Further, ceramics sintered bodies forming the respective air bearing bodies are formed by silicon nitride sintered bodies as described later, whereby abrupt radial impact force during high-speed rotation is supported by the silicon nitride sintered bodies. When impact or vibration is applied from the exterior during high-speed rotation, the outer ring 2 and the cover 4 are to move vertically, i.e., in the thrust direction. Following such vertical movement of the outer ring 2 and the cover 4, the air pressure in the air dome portion 100 fluctuates. Thus, force acts in a direction opposite to that of movement of the outer ring 2 and the cover 4, due to pressure difference between the external atmosphere and the air dome portion 100. Consequently, the thrust-directional movement of the outer ring 2 and the cover 4 caused by vibration or impact which is applied from the exterior during high-speed rotation is suppressed to slight fluctuation. Namely, air pressure fluctuation in the air dome portion 100 absorbs the thrust-directional movement of the outer ring 2 and the cover 4. In this case, the air vent hole 101 has the minimum size required for bleeding air which is introduced along the dynamic pressure producing grooves. It is necessary to suppress the size of the air vent hole 101 to such a degree that entrance/discharge of air cannot follow the vertical movement of the outer ring 2 and the cover 4. Structural Example B Figs. 2A and 2B are conceptual diagrams schematically showing composite bearing structures according to an embodiment of the present invention. As shown in Fig. 2A, a composite bearing body comprises an inner ring 1, an outer ring 2, a lower thrust plate 3 and a cover 4. The respective members 1 to 3 mainly consist of silicon nitride ceramics sintered bodies. The lower thrust plate 3 is provided to be in contact with an end surface of the inner ring 1. The outer ring 2 is provided to keep prescribed clearances with respect to a cylindrical surface of the inner ring 1 and an inner side surface of the lower thrust plate 3. In the case of this structural example, permanent magnets 7 to 10 are provided on cylindrical surfaces of the inner ring 1 and the outer ring 2 which are opposed to each other respectively. Further, thrust dynamic pressure producing grooves 6 are formed in surfaces of the outer ring 2 and the lower thrust plate 3 which are opposed to each other, in the form of a plurality of spirals. The clearance between the inner ring 1 and the outer ring 2 is defined by a first clearance between the ceramic sintered bodies forming the respective members, and a second clearance between the permanent magnets 7 (9) and 8 (10). The second clearance is larger than the first clearance. The cover 4 is bonded to the outer ring 2. A space enclosed with the inner ring 1, the outer ring 2 and the cover 4 defines an air dome portion 100 for buffering thrust-directional impact or vibration. The cover 4 is provided with an air vent hole 101. The air vent hole 101 has the minimum size required for bleeding air which is introduced along the thrust dynamic pressure producing grooves. The size of the air hole 101 is suppressed to such a degree that entrance/discharge of air cannot follow vertical movement of the outer ring 2 and the cover 4. When the outer ring 2 rotates with a rotator in the composite bearing body structured in the aforementioned manner, air is introduced between the outer ring 2 and the lower thrust plate 3 along the thrust dynamic pressure producing grooves 6, whereby the clearance therebetween is kept constant, and a thrust air bearing body is defined. In the radial direction, on the other hand, a constant clearance is kept by repulsion produced between the permanent magnets 7 (9) and 8 (10), whereby rotational accuracy during high-speed rotation is maintained. Even if abrupt radial impact force is applied to the rotator at this time, the inner ring 1 and the outer ring 2 come into contact with each other not between the permanent magnets, but between the silicon nitride sintered bodies of high strength. Therefore, no impactive knock wear phenomenon is caused by contact between the inner ring 1 and the outer ring 2 in sliding following the high-speed rotation. As hereinabove described, not only radial rotational accuracy is maintained and a thrust-directional load is supported, but a composite bearing structure which can also withstand abrupt impact force in the radial direction and the thrust direction is provided in the structural example B. Fig. 2B shows another type of the structural example B. A point which is different from the structure shown in Fig. 2A resides in that a permanent magnet 11 is embedded in the interior of an inner ring 1, and permanent magnets 8 and 10 are provided on outer peripheral cylindrical surfaces of an outer ring 2. In this case, the permanent magnets 8 (10) and 11 produce repulsion through ceramics sintered bodies forming the inner ring 1 and the outer ring 2 respectively. At this time, a clearance between the inner ring 1 and the outer ring 2 is set in consideration of repulsion between the magnets which is required for maintaining prescribed rotational accuracy. Structural Example C Fig. 3 is a conceptual diagram schematically showing a composite bearing structure according to an embodiment of the present invention. As shown in Fig. 3, a composite bearing body comprises an inner ring 1, an outer ring 2, a lower thrust plate 3 and a cover 4. The cover 4 is bonded to the outer ring 2. The cover 4 has an air vent hole 101. The air vent hole 101 has the minimum size required for bleeding air which is introduced by thrust dynamic pressure producing grooves 6. The respective members 1 to 3 consist of silicon nitride ceramics sintered bodies. The inner ring 1 has a cylindrical shape, to be capable of enveloping an outer peripheral surface of a prescribed shaft. The lower thrust plate 3 is provided to be in contact with an end surface of the inner ring 1. The outer ring 2 is provided to keep prescribed clearances with respect to the lower thrust plate 3 and the inner ring 1 respectively. Spiral thrust dynamic pressure producing grooves 6 are formed in surfaces of the outer ring 2 and the lower thrust plate 3 which are opposed to each other. The clearance between the inner ring 1 and the outer ring 2 is set at a small value, to be capable of maintaining prescribed rotational accuracy during high-speed rotation. Thus, a thrust-directional load is supported by an air bearing body defined in the aforementioned manner, and radial rotational accuracy is maintained by the small clearance between the inner ring 1 and the outer ring 2. Further, abrupt radial impact force during high-speed rotation is supported by the silicon nitride ceramics sintered bodies of high strength forming the inner ring 1 and the outer ring 2. Further, thrust-directional impact or vibration during high-speed rotation is absorbed by an air dome portion 100 which is enclosed with the inner ring 1, the outer ring 2 and the cover 4. Structural Example D Fig. 4 is a conceptual diagram schematically showing a composite bearing structure according to an embodiment of the present invention. As shown in Fig. 4, a composite bearing body comprises an inner ring 1, an outer ring 2, a lower thrust plate 13 and a cover 4. The inner ring 1 and the outer ring 2 are mainly formed by silicon nitride ceramics sintered bodies. The inner ring 1 has a cylindrical shape, to be capable of enveloping an outer peripheral surface of a prescribed shaft. A plurality of V-shaped radial dynamic pressure producing grooves 5 are formed in the cylindrical surface of the inner ring 1. The lower thrust plate 13 consists of a permanent magnet. A ring-shaped permanent magnet 12 is provided on a surface of the outer ring 2 which is opposed to the lower thrust plate 13. The cover 4 is bonded to the outer ring 2. The cover 4 has an air vent hole 101. The air vent hole 101 has the minimum size required for bleeding air which is introduced by the radial dynamic pressure producing grooves 5. When the outer ring 2 rotates with a rotator in the composite bearing body formed in the aforementioned manner, air is introduced between the inner ring 1 and the outer ring 2 along the radial dynamic pressure producing grooves 5, so that a clearance therebetween is kept constant. Thus, an air bearing body is defined, whereby radial rotational accuracy is maintained. Further, the permanent magnet 12 of the outer ring 2 and the lower thrust plate (permanent magnet) 13 produce repulsion, whereby a prescribed clearance is kept between the outer ring 2 and the lower thrust plate 13. Thus, a thrust load during high-speed rotation is supported. Further, the inner ring 1 and the outer ring 2 are formed by silicon nitride sintered bodies of high strength, whereby no knock wear phenomenon takes place even if abrupt impact force is applied during high-speed rotation and the inner ring 1 comes into contact with the outer ring 2. In addition, thrust-directional impact or vibration during high-speed rotation is absorbed by the action of an air dome portion 100 which is enclosed with the inner ring 1, the outer ring 2 and the cover 4. According to the structural example D, as hereinabove described, rotational accuracy is maintained and the thrust-directional load is supported during high-speed rotation, while a composite bearing body which can also withstand impact force during high-speed rotation is provided. Structural Example E Figs. 5A and 5B are conceptual diagrams showing structures of composite bearing bodies according to an embodiment of the present invention. As shown in Fig. 5A, a composite bearing body comprises an inner ring 1, an outer ring 2, a lower thrust plate 3 and a cover 4. The cover 4 is bonded to the outer ring 2. The respective members 1 to 3 are mainly formed by silicon nitride ceramics sintered bodies. A point which is different from the structural example B (Fig. 2A) resides in a structure of the bearing body in the thrust direction. Permanent magnets 15 and 16 are provided on surfaces of the outer ring 2 and the lower thrust plate 3 which are opposed to each other. The permanent magnet 15 is provided to be also opposed to a permanent magnet 14 of the inner ring 1. Thus, a thrust-directional load is also supported by a magnetic bearing body. According to this structural example E, radial rotational accuracy is maintained by the magnetic bearing body, a thrust-directional load is supported by the magnetic bearing body, and radial impact force is supported by the ceramics sintered bodies of high strength. Thrust-directional impact or vibration is absorbed by suppressive force which is caused by air pressure fluctuation of an air dome portion 100 which is enclosed with the inner ring 1, the outer ring 2 and a cover 4. Fig. 5B shows another type of the structural example E. A point which is different from the structural example B (Fig. 2B) resides in that a thrust-directional load is supported not by an air bearing body, but by a magnetic bearing body. Namely, a permanent magnet 17 of a lower thrust plate 3 is provided to be opposed to a permanent magnet 10 of an outer ring 2. Structural Example F Fig. 6 is a conceptual diagram schematically showing the structure of a composite bearing body according to an embodiment of the present invention. As shown in Fig. 6, the composite bearing body comprises an inner ring 1, an outer ring 2, a lower thrust plate 13 and a cover 4. The cover 4 is bonded to the outer ring 2. The inner ring 1 and the outer ring 2 are mainly formed by silicon nitride ceramics sintered bodies. The inner ring 1 has a cylindrical shape, to be capable of enveloping an outer peripheral surface of a prescribed shaft. The lower thrust plate 13 is provided to be in contact with an end surface of the inner ring 1. The lower thrust plate 13 consists of a permanent magnet. A permanent magnet 12 is provided on a surface of the outer ring 2 which is opposed to the lower thrust plate 13. A clearance between the inner ring 1 and the outer ring 2 is set at a small value to be capable of maintaining prescribed rotational accuracy during high-speed rotation. Thus, a thrust-directional load is supported by repulsion produced by the permanent magnet 12 and the lower thrust plate (permanent magnet) 13, while radial impact force is supported by the silicon nitride sintered bodies of high strength forming the inner ring 1 and the outer ring 2. Further, thrust-directional impact or vibration is absorbed by the action of an air dome portion 100 which is enclosed with the inner ring 1, the outer ring 2 and the cover 4. Summarizing the aforementioned structural examples A to F, it is possible to attain simplification of structures in the magnetic bearing bodies of the structural examples D to F as compared with the air bearing bodies of the structural examples A to C, as to a support structure for a thrust-directional load. When a thrust-directional load is supported by an air bearing body, a constant area is required in order to ensure lifting power. Further, it is possible to produce lifting power from starting of rotation according to a magnetic bearing body, while lifting power is not produced according to an air bearing body until a constant rotational frequency is exceeded. As means for improving radial rotational accuracy, it is effective to prepare a radial bearing body from an air bearing body (structural example A or D), or also employ a magnetic bearing body (structural example B or E). Also when a bushing type slide bearing body is employed, it is possible to improve rotational accuracy by setting a clearance (gap) between an inner ring and an outer ring at a small value (structural example C or F). Every structural example can satisfy radial impact resistance. In every structural example, no seizure is caused even if impact force is applied during high-speed rotation to bring the inner ring and the outer ring into contact with each other. Particularly when silicon nitride ceramics sintered bodies of high strength are employed, excellent bearing characteristics can be maintained since neither breaking nor chipping is caused. Description is now made on method of manufacturing a silicon nitride sintered body as a material for a radial impact resistance member in a composite bearing structure according to Example of the present invention. Raw material powder of Si 3 N 4 having a mean particle size of 0.3 µm, particle size distribution of 3σ = 0.20 µm, an α crystallization rate of 96.5 % and an oxygen content of 1.4 weight % was prepared. 92 weight % of this raw material powder of Si 3 N 4 , 4 weight % of Y 2 O 3 powder of 0.8 µm in mean particle size, 3 weight % of Al 2 O 3 powder of 0.5 µm in mean particle size, and 1 weight % of AlN powder of 1.0 µm in mean particle size were wet-blended in ethanol for 100 hours through a ball mill. Thereafter mixed powder obtained by drying was CIP (cold isostatic press) molded under a pressure of 5000 kg/cm 2 . The compact as obtained was held under a nitrogen gas atmosphere of 1 atm. at a temperature of 1600°C for 4 hours. Further, sintering treatment was carried out at a temperature of 1750°C for 6 hours, to obtain a sintered body. Thereafter HIP (hot isostatic press) treatment of 2 hours was carried out on this sintered body at a temperature of 1700°C in a nitrogen gas atmosphere of 1000 atm. In the sintered body of silicon nitride thus obtained, a mean major axis particle size was not more than 5 µm, an aspect ratio of its crystal grains was at least 4, and the maximum major axis size was not more than 15 µm. This sintered body had mechanical properties of JIS three-point bending strength of at least 80 kg/mm 2 and a fracture toughness value of at least 5 MPa·m ½ . Test pieces were cut out from the sintered body as obtained, and subjected to evaluation of wear resistance in accordance with an Amsler abrasion test. Fig. 7A is a graph showing the relations between products (P•V) of loads applied to the samples and rotational speeds, and wear rates. Referring to Fig. 7A, respective curves a to g show measurement results obtained by the following samples: a: Inventive Example A silicon nitride sintered body obtained by the aforementioned manufacturing method, having linear density of crystal grains of 40 per length of 30 µm, a volume rate of grain boundary phases of 8 volume %, porosity of 0.05 %, and a maximum pore diameter of 8 µm. b: Comparative Example A silicon nitride sintered body obtained by the aforementioned manufacturing method, having linear density of crystal grains of 30 per length of 30 µm, a volume rate of grain boundary phases of 16 volume %, porosity of 3.2 %, and a maximum pore diameter of 22 µm. c, d, e: Conventional Examples Samples of a silicon nitride sintered body shown in Yogyo Kyokai-Shi, 1985, Vol. 93, pp. 73 to 80 (Fig. 3 in particular). f: Commercially Available Alumina Sintered Body g: Commercially Available Silicon Carbide Sintered Body The Amsler abrasion test was made by fixing two ring-shaped samples (16 mm x 30 mm x 8 mm) to a rotary shaft of a tester in such a state that circumferential surfaces thereof were correctly in contact with each other, applying a prescribed load and driving the rotary shaft at a prescribed rotational frequency thereby carrying out a sliding test of about 100000 revolutions, and thereafter measuring degrees of reduction in weight of the two samples. As clearly understood from Fig. 7A, the sintered body according to the present invention is extremely excellent in wear resistance. Fig. 7B shows the values of P•V (kg•cm/s) in Fig. 7A in terms of those per unit area of a sliding surface. Consider that a sintered body which is prepared in the aforementioned manner is exposed to sliding in high-speed rotation of a spindle motor for a hard disk driver, for example. A single hard disk has a thickness of about 2 mm in a disk of about 50 to 200 mm. Further, a plurality of hard disks may be overlapped with each other and driven to rotate. The weight of such a hard disk is about 20 to 1000 g. Assuming that the diameter of a bearing body supporting the aforementioned hard disk is 10 mm and the rotational frequency of the hard disk is 2000 to 50000 r.p.m., therefore, its peripheral speed is 100 to 2600 cm/s. Assuming that a bearing area is 3.14 cm 2 at this time, the value of P•V (kg/cm•s) is 1 to 828 kg/cm·s. Refer to Fig. 7B with respect to P•V values in this range. The sintered body according to the present invention exhibits low wear rates with respect to P•V values of a wide range of 1 to 600 kg/cm•s, and can sufficiently withstand sliding in high-speed rotation of the aforementioned hard disk. Namely, such a case that the sintered body regularly slides during high-speed rotation is considered in the aforementioned example. Also in that case, the sintered body according to the present invention exhibits low wear rates with respect to P•V values of a wide range, whereby the sintered body according to the present invention can sufficiently withstand sliding even if abrupt impact force is applied during high-speed rotation to cause sliding following the high-speed rotation. Then, spindle motors were prepared as to the respective cases of the aforementioned structural examples F and C, and subjected to evaluation of changes in thrust-directional rotational accuracy upon application of impact during high-speed rotation. Fig. 8 is a schematic diagram showing the structure of a spindle motor employed for the aforementioned evaluation. Referring to Fig. 8, a bearing of the structural example F is mounted on a portion of a bearing 201 of the spindle motor. A rotating portion of the bearing is mounted on a hub 204, while a fixed portion of the bearing is mounted on a mounting base material 205. A rotor magnet 203 which is mounted on the hub 204 is provided to be opposed to a motor coil 202 which is provided on the mounting base material 205. In order to evaluate bearing performance of the structural example F, three types of bearings of Example (A) and comparative examples (B) and (C) shown in Figs. 9A, 9B and 9C were mounted on the portion of the bearing 201 of the spindle motor to carry out an evaluation test. The evaluation test was carried out by applying impact of 0.3 N·m from the exterior while making the hub 204 rotate at 30,000 r.p.m. An upper surface of the hub 204 was measured with a laser displacement gauge, to measure amounts of upward displacement of the hub 204. Further, fluctuation widths of motor current quantities corresponding to torque fluctuation were also measured. In comparative example (B), a thrust plate 40 was provided on an upper portion of the bearing, and a clearance between an outer ring 2 and the thrust plate 40 was set at 40 µm, as shown in Fig. 9B. In the structure of comparative example C (Fig. 9C), an upper portion of the bearing was opened. In the three bearing structures of (A) to (C), the outer diameters of the inner rings 1 were set at 10 mm, and radial clearances were set at 10 µm. Results of the aforementioned evaluation are shown in Table 2. Amount of Upward Displacement of Hub (µm) Fluctuation Width of Motor Current (A) Example (A) 2 0 Comparative Example (B) 40 0.9 Comparative Example (C) 150 0.2 In the bearing structure of Example (A), the amount of upward displacement of the hub was extremely small even if external impact was applied, and the fluctuation width of the motor current corresponding to torque fluctuation was so small that the same was substantially negligible. In the bearing structure of comparative example (B), on the other hand, the amount of upward displacement of the hub was 40 µm, due to suppression by the thrust plate. However, fluctuation of rotational torque was increased by contact between the outer ring 2 and the thrust plate 40. In the bearing structure of comparative example (C), an extremely large amount of displacement of 150 µm was observed since no means for suppressing upward displacement of the hub was provided. From the aforementioned results, it has been proved that high rotational accuracy can be maintained in the thrust direction with respect to disturbance such as impact from the exterior by providing the air dome portion 100. In the bearing structure of Example (A), further, an impact test similar to the above was made while changing the height L of the air dome portion 100 as 0.5 to 5 mm. The results are shown in Table 3. L(mm) Amount of Upward Displacement of Hub (µm) 0.5 0.8 1 2 5 9 As obvious from the results of Table 3, it is understood that the effect of suppressing thrust-directional displacement is increased as the volume of the air dome portion 100 is reduced. This is conceivably because the rate of pressure change in the air dome portion 100 is increased as the volume of the air dome portion 100 is reduced. Then, the bearing of the structural example C was mounted on the portion of the bearing 201 of the spindle motor shown in Fig. 8. In order to evaluate bearing performance of the structural example C, three types of bearings of Example (A) and comparative examples (B) and (C) shown in Figs. 10A, 10B and 10C were mounted. Impact of 0.5 N·m was applied from the exterior while making the hub 204 rotate at 20,000 r.p.m. At this time, amounts of vertical movement of the upper surface of the hub 204 were measured with a laser displacement gauge, to measure the amounts of upward displacement of the hub 204. Fluctuation widths of motor current quantities corresponding to torque fluctuation were also measured at the same time. In the bearing structure of comparative example (B) (Fig. 10B), a thrust plate 40 was provided on an upper portion of the bearing, and a clearance between an outer ring 2 rotating at 20,000 r.p.m. and the thrust plate 40 was set at 50 µm. In the bearing structure of comparative example (B), an upper portion of the bearing was opened. In the three bearing structures of Figs. 10A, 10B and 10C, outer diameters of inner rings 1 were set at 15 mm, and radial clearances were set at 15 µm. In the bearing structure of Example (A) (Fig. 10A), an air vent hole 101 of 0.5 mm was provided in a cover 4. Results of the aforementioned evaluation are shown in Table 4. Amount of Upward Displacement of Hub (µm) Fluctuation Width of Motor Current (A) Example (A) 3 0 Comparative Example (B) 50 0.8 Comparative Example (C) 180 0.1 As obvious from Table 4, the amount of upward displacement of the hub was extremely small in the bearing structure of Example (A) even if impact was applied from the exterior, and the fluctuation width of the motor current corresponding to torque fluctuation was also so small that the same was substantially negligible. In the bearing structure of comparative example (B), on the other hand, the amount of upward displacement of the hub was 50 µm, due to suppression by the thrust plate. However, fluctuation of the motor current corresponding to rotational torque was increased by contact between the outer ring 2 and the thrust plate 40. In the bearing structure of comparative example (C), an extremely large amount of displacement of 180 µm was observed since no means for suppressing upward displacement of the hub was provided. From the aforementioned results, it has been proved that high rotational accuracy can be maintained in the thrust direction with respect to disturbance such as impact from the exterior by providing the air dome portion 100. In the bearing structure of Example (A) (Fig. 10A), further, an impact test similar to the above was made while changing the height L of the air dome portion 100 as 0.3 to 3 mm. The results are shown in Table 5. L (mm) Amount of Upward Displacement of Hub (µm) 0.3 1 1 3 3 10 As obvious from the results of Table 5, it is understood that the effect of suppressing thrust-directional displacement of the hub is increased as the volume of the air dome portion 100 is reduced. This is conceivably because the rate of pressure change in the air dome portion 100 is increased as the volume of the air dome portion 100 is reduced. In the bearing structure of Example (A) (Fig. 10A), further, the diameter D of the air vent hole 101 provided in the cover 4 was changed as 0.5 to 5 mm, to carry out an impact test similar to the above. The results are shown in Table 6. At this time, it was set that L = 1 mm. D (mm) Amount of Upward Displacement of Hub (µm) 0.5 3 1 6 5 40 As shown in Table 6, the effect of suppressing the amount of upward displacement of the hub appeared also when the diameter (D) of the air vent hole was 5 mm, as compared with the case of opening the upper portion of the bearing as in comparative example (C). In order to further suppress the amount of upward displacement of the hub, however, it is preferable to suppress the diameter of the air vent hole to the minimum size required for bleeding air which is introduced along the thrust dynamic pressure producing grooves. While a bearing body for a spindle motor which is applied to a hard disk driver or a photomagnetic disk driver has been shown as Example of the inventive composite bearing body, the present invention is not restricted to this. The inventive composite bearing body is also applicable to a turbine bearing for a supercharger rotating at a high speed of 80000 to 150000 r.p.m., a bearing for a turbine or a compressor rotating at a high speed of 20000 to 30000 r.p.m., a high rotation bearing employed for a turbo-pump for a rocket engine, or a bearing employed for a machine tool such as a CNC ultraprecise lathe, a cylindrical working ultraprecise lathe, an ultraprecise surface grinder or the like. Industrial Availability As hereinabove described, the inventive composite bearing structure is suitable for supporting a rotator rotating at a high speed and excellent in impact resistance and vibration resistance, whereby the same is applicable to a bearing body for a spindle motor which is employed for a hard disk driver or a photomagnetic disk driver.

Claims: A composite bearing structure for supporting a rotator having a prescribed load and rotating at a high speed, said composite bearing structure comprising: radial bearing means (1, 2) including a ceramics sintered body for supporting radial impact force being applied to said rotator during rotation and for maintaining radial rotational accuracy of said rotator; axial bearing means for supporting an axial load being applied to said rotator in a state keeping a prescribed clearance with said rotator; wherein said axial bearing means includes a gas dome portion (100) being confined in a semi-closed state by a rotating member (4) rotating with said rotator and a fixed member (1) being axially opposed to said rotating member for maintaining thrust-directional rotational accuracy of said rotator, characterized in that said ceramics sintered body includes a silicon nitride ceramics sintered body, wherein said silicon nitride ceramics sintered body includes a sintered body of silicon nitride containing crystal grains having linear density of at least 35 per length of 30 µm with a volume rate of grain boundary phases of not more than 15 volume %, and containing pores of not more than 20 µm in maximum diameter, the content of said pores being not more than 3 %. A composite bearing structure in accordance with claim 1, wherein said gas dome portion (100) comprises a cover (4) provided with an air vent hole (101). A composite bearing structure in accordance with claim 1, wherein said axial bearing means includes dynamic gas bearing means (6) for supporting said load by a gas pressure being produced by rotation of said rotator. A composite bearing structure in accordance with claim 3, wherein said dynamic gas bearing means includes a rotating member (2) rotating with said rotator, and a fixed member (3) being opposed to said rotating member while axially keeping said clearance. A composite bearing structure in accordance with claim 4, wherein said rotating member and said fixed member include ceramics sintered bodies, either one of said rotating member and said fixed member being provided with a groove (6) for introducing a gas into said clearance for producing said gas pressure. A composite bearing structure in accordance with claim 1, wherein said axial bearing means includes magnetic bearing means (12, 13; 10, 17; 15, 16) for supporting said load through magnetic attraction or repulsion. A composite bearing structure in accordance with claim 6, wherein said magnetic bearing means includes a rotating member (2) rotating with said rotator, and a fixed member (3; 13) being opposed to said rotating member while axially keeping said clearance. A composite bearing structure in accordance with claim 7, wherein said rotating member and said fixed member include magnets (12, 13; 10, 17; 15, 16) for producing magnetic attraction or repulsion in said clearance. A composite bearing structure in accordance with claim 1, wherein said radial bearing means includes a ceramics sintered body for maintaining said rotational accuracy in a state keeping a prescribed clearance with said rotator. A composite bearing structure in accordance with claim 1, wherein said radial bearing means includes dynamic pressure gas bearing means (5) for maintaining said rotational accuracy by a gas pressure being produced by rotation of said rotator. A composite bearing structure in accordance with claim 10, wherein said dynamic pressure gas bearing means includes a rotating member (2, 4) rotating with said rotator, and a fixed member (1) being opposed to said rotating member while radially keeping said clearance. A composite bearing structure in accordance with claim 11, wherein said rotating member and said fixed member include ceramics sintered bodies, either one of said rotating member and said fixed member being provided with a groove (5) for introducing a gas into said clearance for producing said gas pressure. A composite bearing structure in accordance with claim 1, wherein said radial bearing means includes magnetic bearing means (8, 10, 11; 7, 8, 9, 10; 7, 8, 14, 15) for maintaining said rotational accuracy through magnetic attraction or repulsion in a state keeping a prescribed clearance with said rotator. A composite bearing structure in accordance with claim 13, wherein said magnetic bearing means includes a rotating member (2) rotating with said rotator, and a fixed member (1) being opposed to said rotating member while radially keeping said clearance. A composite bearing structure in accordance with claim 14, wherein said rotating member and said fixed member include magnets (8, 10, 11; 7, 8, 9, 10; 7, 8, 14, 15) for producing magnetic attraction or repulsion in said clearance. A composite bearing structure in accordance with claim 15, wherein said rotating member and said fixed member consist of said ceramic sintered bodies, said magnets (11) being embedded in the interior of said ceramics sintered bodies. A composite bearing structure in accordance with claim 1, wherein said rotating member includes a first rotating portion (2) being opposed to said fixed member (1) while radially keeping a clearance, and a second rotating portion (4) being opposed to said fixed member (1) while axially interposing said gas dome portion (100), and fixed to said first rotating portion (2). A composite bearing structure in accordance with claim 17, wherein said axial bearing means includes dynamic pressure gas bearing means (6) introducing a gas into said prescribed clearance by rotation of said rotator, for supporting said load by a gas pressure as produced, said second rotating portion (4) having a hole (101) for discharging said gas being introduced into said prescribed clearance by said dynamic pressure gas bearing means. A composite bearing structure in accordance with claim 17, wherein said radial bearing means includes dynamic pressure gas bearing means (5) introducing a gas into said prescribed clearance by rotation of said rotator, for maintaining said rotational accuracy by a gas pressure as produced, said second rotating portion (4) having a hole (101) for discharging said gas being introduced into said prescribed clearance by said dynamic pressure gas bearing means.

Inventors: KATAYAMA TETSUYA, KOMURA OSAMU, YAMAKAWA AKIRA, KATAYAMA, TETSUYA, KOMURA, OSAMU, YAMAKAWA, AKIRA
Applicants: SUMITOMO ELECTRIC INDUSTRIES, SUMITOMO ELECTRIC INDUSTRIES, LIMITED
Application year: 1994
Main classification: F16C32
IPC codes: F16C17, F16C39, F16C33, F16C32